Electronic welding station with AC and reversible polarity DC outputs

ABSTRACT

An electronic welding station suitable for use with a dedicated power supply or for use in a system wherein a plurality of electronic welding stations are powered from a central power supply. The welding station provides for a positive ground mode, a negative ground mode, and two AC output modes of operation. Most components are used for all modes of operation. The main transistor bank (57) and an auxiliary transistor bank (64) are used in an emitter follower configuration for negative ground mode, a common emitter configuration for positive ground mode, and, in conjunction with an inductor/transformer (67), in a push-pull configuration for both AC output modes of operation. The inductor/transformer (67) is configured as an inductor for DC output operation and, by the insertion of a plug (67E), as a transformer for AC output operation. Selection of the positive ground mode, the negative ground mode, or an AC output mode is achieved by the use of a multi-pole switch (25) and a jumper (90). The result is an extremely versatile electronic welding station.

Technical Field

The present invention relates to electronic welding stations and, inparticular, to an electronic welding station which can provide an ACoutput and a reversible polarity DC output.

BACKGROUND OF THE INVENTION

Electronic welding stations, such as those disclosed in U.S. Pat. Nos.4,301,355 to Kimbrough et al., 4,349,720 to Makimaa, 4,409,465 toYamamoto et al., 4,427,874 to Tabata et al., and 4,716,274 to Gilliland,generally have two output terminals, one of which is a direct connectionto the DC power supply, and the other of which is connected to the powersupply through a switching transistor and other components. Generally,the welding torch is connected to the switched output terminal. If onlya single welding station is being used or if each welding station of amultitude of welding stations has an independent power supply then thepolarity of the welding torch, with respect to the workpiece, can beselected by connection of the welding torch connector to, as desired,the positive output terminal or the negative output terminal of thewelding station, and by connection of the workpiece to the other outputterminal. However, when multiple welding stations are operated from acentral power supply, such as disclosed in U.S. Pat. No. 4,716,274,simple reversal of the welding torch and workpiece connections on all ofthe welding stations will not achieve the desired result because theworkpieces, which are generally interconnected, are connected to theswitched output terminals. The result is that the switched outputterminals are all operating in parallel and the desired control over thewelding operation cannot be achieved. Therefore, there is a need for anelectronic welding station which can be configured so that a selectedone of the welding torch and the workpiece can be directly connected tothe central power supply and the other connected via the switchingtransistor.

Electronic welding stations, such as those described in the abovementioned U.S. patents, generally provide a unipolar output. That is,the output of the electronic welding station is DC or pulsed DC and isalways of the same polarity. These electronic welding stations do notprovide an AC output. However, an AC output is desirable, especially foraluminum-TIG (tungsten inert gas) welding. Therefore, there is a needfor an electronic welding station which can provide an AC output.

AC welding stations have been designed but it has been necessary to haveboth a conventional DC welding station, with its DC output, and an ACwelding station, so that aluminum-TIG welding can be performed. Thisdoubles the number of welding stations, which greatly increases thetotal cost and complicates maintenance and logistics. Therefore, thereis a need for an electronic welding station which can provide either aDC output or an AC output, as selected.

In an electronic welding station which can perform both DC and ACwelding operations it is desired to maximize the number of componentsthat are used for both DC operations and AC operations and to minimizethe number of components, especially larger and/or more expensivecomponents, that are used for only one mode of operation. It istherefore desirable to be able to use the output inductor in a DC outputwelding station, for the transformer in an AC output welding station.However, the open construction of a typical inductor core generallyprovides insufficient coupling for use as a transformer and the closedconstruction of a typical transformer core provides too high aninductance for use in most DC-output welding operations. Therefore,there is a need for a component which can be selectively configured tooperate as either an inductor or as a transformer, as required for theparticular operation to be performed.

In conventional DC output electronic welding stations, the switchingtransistor is either in an on state wherein the full power supplyvoltage is applied to the switched output terminal, or in an off statewherein the power supply is isolated from the switched terminal. Inthese welding stations, the welding current is controlled by varying thepulsewidth (the on time of the switching transistor). However, thistechnique is generally not applicable to AC output welding stationssince changing the pulsewidth for one polarity of the output waveformcauses an opposite change in the pulsewidth of the other polarity of theoutput waveform. Therefore, the full power supply voltage, of onepolarity or the other, would always be applied to the output terminal.Therefore, there is a need for a method for controlling the weldingcurrent of an AC output electronic welding station.

Electronic welding stations, in order to reduce manufacturing andmaintenance costs, generally use a bank of switching transistors, ratherthan a single switching transistor. Some of the transistors will beeasily accessible and, in the event of failure, can be easily replaced.However, some transistors will not be as accessible and, in order thatreplacement may be accomplished, the entire bank of switchingtransistors may have to be removed and/or other components or circuitsof the electronic welder may have to be removed. In the latter case, asubstantial amount of time may be required to replace the defectivetransistor, thus increasing the maintenance costs, and the removal ofcomponents other than the one which is to be replaced provides anincreased occasion for misassembly and subsequent destruction of theelectronic welder. Therefore, it is highly desirable to design anelectronic welding station so that a transistor which fails ispreferably a transistor which is easily replaced.

For a specified weld type, gas type, gas flow rate, welding rod materialand diameter, torch travel speed, and work material type and thickness,there will be a range of values for the pulse frequency, a range ofvalues for the pulsewidth, and a range of values for the wire feed speedthat will produce an acceptable weld. Of course, for cost and efficiencyreasons, it is desirable to perform a welding operation as rapidly as ispossible while still producing a quality weld. To achieve this, wirefeed speed and arc current should generally be at the high end of theallowable range of values, subject, of course, to adjustment based uponthe skill of the individual welder. Some electronic welding stationshave a plurality of accessible controls so that the wire feed speed, thepulse frequency, the pulsewidth (and the arc current), etc., can beindividually varied. Other electronic welding stations try to sense thearc current or the arc voltage and adjust the wire feed speed and otherparameters so as to maintain a constant arc current or arc voltage orrespond to the arc current or arc voltage. However, the first type ofelectronic welding station cannot be rapidly adjusted because theoperator may have three to five knobs to reposition while trying toobtain the desired arc. The second type of electronic welding stationfrequently produces less than desirable results because some of theparameters are not a precise function or a linear function of the arccurrent, the arc voltage, any particular individual factor, or anyparticular group of factors. Therefore, there is a need for anelectronic welding station which minimizes the number of parameters thatthe operator has to select and which adjusts other dependent parametersso as to produce an optimum weld in light of the parameters selected bythe operator.

It is desirable to monitor the arc voltage to determine whether the archas been struck so as to allow the electronic welding station to use oneset of parameters which are desirable for striking an arc and to useanother set of parameters which are desirable for conducting the weldingoperation. Therefore, it is desirable to provide an arc detectioncircuit which operates with electronic welding stations which provideboth positive ground and negative ground DC outputs and AC outputs.

SUMMARY OF THE INVENTION

The present invention is an electronic welding station which overcomesthe above listed disadvantages of the prior art and, furthermore,overcomes these disadvantages in a manner which minimizes themanufacturing and maintenance costs of the electronic welding station.Broadly stated, the present invention is an electronic welding stationwhich can be selectively configured to operate with either a positiveground or a negative ground by simply setting the output polarity switchto the desired position. In the negative ground position, the negativepower supply lead is connected to the negative output terminal of thewelding station and the positive power supply lead is connected to thepositive output terminal of the welding station through a bank ofswitching transistors and one or more inductors and resistors. In thepositive ground position, the positive power supply lead is directlyconnected to the positive output terminal of the welding station and thenegative power supply lead is connected through the same bank ofswitching transistors, inductors and resistors to the negative outputterminal. A switching system changes the interconnections between thebank of switching transistors, inductors and resistors, power supplyleads, and output terminals, so that the most or all of same componentsare used regardless of whether positive ground or negative groundoperation is selected.

Therefore, one object of the present invention is to provide anelectronic welding station which can selectively operate in either apositive ground or a negative ground configuration.

It is another object of the present invention to provide an electronicwelding station which uses the same components for positive groundoperation and negative ground operation.

The present invention also provides an electronic welding station whichprovides an AC output for aluminum-TIG welding. The present inventionincludes a device for controlling the frequency of the AC output and forcontrolling the duration of the pulse on one polarity of the AC cycle ascompared to the duration of the pulse on the other polarity of the ACcycle so that square wave and asymmetrical wave outputs can be selected.To minimize construction and maintenance costs, the electronic weldingstation described by the present invention uses most of the samecomponents that are required for an electronic welder having only a DCoutput and minimizes the number of additional components that arerequired.

Therefore, it is an object of the present invention to provide anelectronic welding station which provides both AC and DC outputs.

It is another object of the present invention to provide an electronicwelding station, having both AC and DC outputs, which uses the samecomponents to provide the AC output and the DC output.

The present invention also includes a novel arrangement whereby an ironcore, having several windings, is used as an inductor for DC operationby the removal of a section of the core, and is used as a transformerfor AC operation by insertion of the section into the core. When thesection is removed, a substantial air gap is introduced into the core.This reduces the coupling between the windings on the core and alsoreduces the inductance of each of the windings so that one of thewindings can be used to provide the relatively small inductance requiredfor DC operation. When the section is reinserted into the core, the airgap is eliminated, thereby increasing the inductance of each of thewindings and substantially increasing the coupling between the windingsso that the device can now be used as a transformer for AC outputoperation.

Therefore, it is an object of the present invention to provide a devicewhich can be used as an inductor for DC output operation and as atransformer for AC output operation.

It is a further object of the present invention to provide a devicewhich can be configured to operate as an inductor or as a transformer bythe removal or insertion of a section of the core of the device.

In DC output operation, the output current can be controlled by varyingthe on time and the off time of the transistors in the switching bank.This is not effective for some types of AC output operation becausereducing the duration of one polarity of the AC output waveform simplyincreases the duration of the other polarity of the AC output waveform.In the present invention, the output power or current in AC outputoperation is varied by adjusting the degree of saturation of the outputtransformer, the reactance of a variable reactor connected in serieswith the output, or both. A higher degree of saturation for the outputtransformer and a higher reactance for the variable reactor both resultin less welding current, whereas a lower degree of saturation for theoutput transformer and a lower reactance for the variable reactor bothresult in more welding current. In the present invention, the reactanceof the output transformer or the variable reactor is varied bycontrolling the degree of saturation of the core of the outputtransformer or the variable reactor. One or more variable DC powersupplies control the degree of saturation of the output transformer orthe variable reactor. If more saturation (less reactance) is desired,the output of the variable DC power supply is increased. If lesssaturation (more reactance) is desired, the output of the DC powersupply is decreased.

Therefore, it is an object of the present invention to provide a meansfor controlling the welding current in an AC output operation by varyingthe degree of saturation, and therefore the coupling, of the outputtransformer.

It is a further object of the present invention to provide a means forcontrolling the welding current in an AC output operation by means of avariable reactor.

It is a further object of the present invention to control the degree ofsaturation of the core of an output transformer or a variable reactor byuse of a variable DC power supply.

The present invention also provides an electronic welding station withreduced maintenance costs. The biasing circuitry for one transistor isslightly different from the biasing circuitry for the rest of thetransistors in the switching bank so that this transistor tends to moveout of saturation or near saturation before any of the othertransistors. When a welding condition occurs which tends to cause anexcessive current to flow through the transistors, this transistor willheat up more rapidly than any of the rest of the transistors. This tendsto cause the failure of this transistor in preference to the failure ofthe remaining transistors. Because of protection circuitry in theelectronic welding station, the failure of the preferential transistorremoves the drive power from the remaining transistors so that they areprotected. In the present invention, the preferential transistor ismounted in the electronic welding station in a position where it isreadily accessible for removal and replacement. This results in asubstantial savings in maintenance costs because it is not necessary toremove and replace other components in order to obtain access to thistransistor.

Therefore, it is an object of the present invention to reducemaintenance costs by mounting one of the transistors in the switchingbank in a position which is readily accessible and biasing thistransistor so that it fails in preference to any of the othertransistors in the switching bank.

It is another object of the present invention to provide an apparatuswhich causes one transistor to fail in preference to other transistorswhich are connected in parallel.

The present invention also provides a method and an apparatus foroptimizing a welding operation. The method contemplates selecting anindependent parameter, recording the values for this independentparameter and corresponding values for a set of other parameters whichproduce optimum welding results, and providing a control system whichvaries these other parameters as a function of the selected parameter sothat the operator need only adjust a single parameter control. Oneembodiment of the present invention provides an apparatus which has aplurality of ganged potentiometers, the tapers and tap points of thesepotentiometers being designed so that the parameters which they controltrack the selected parameter so as to produce optimum welding results.Another embodiment of the present invention contemplates amulti-position switch which selects a desired value for the selectedparameter, a memory which has optimum values for the other parametersgiven the selected value for the selected parameter, and a processorwhich adjusts the parameters in accordance with the values stored in thetable in the memory. The method and apparatus disclosed serve to reducethe number of controls that the operator must adjust. This results in ahigher efficiency for the operator and better welding results.

Therefore, it is an object of the present invention to improve theoperator's efficiency and improve the quality of the weld by reducingthe number of parameters that the operator must select.

Since different types of welding operations, different types andthicknesses of materials, different shielding gases, etc., affect theinterdependence between the various parameters, the present inventionprovides a plurality of modules. Each module is designed for aparticular type of welding operation. Therefore, the parameter valuesused for a particular welding operation will be the optimum parametersfor that type of welding operation and need not be a compromise betweenthe values for several different welding operations. In one embodiment,each module contains a different set of ganged potentiometers, thepotentiometers having the tapers and taps necessary to achieve thedesired results for a specified type of welding operation. In anotherembodiment, each module contains a memory which has a table of theoptimum values for the various parameters. The use of modules allows theoptimum welding parameters for a plurality of different weldingoperations to be selected by the use of a single parameter selectioncontrol.

Therefore, it is an objection of the present invention to provide anelectronic welding station which utilizes a plurality of modules toachieve optimum welding results for a plurality of different weldingoperations, each module varying the parameters as necessary to achieveoptimum welding results for a specified type of welding operation.

The presence or absence of the arc can be determined by monitoring theoutput voltage. Depending upon the operating mode selected, the outputvoltage may be positive, negative, or AC with respect to the workpiece.The present invention provides an arc detector circuit which monitorsthe absolute value of the output voltage. The circuit considers the arcto have been struck when the absolute value of the output voltage dropsbelow a predetermined reference value. Furthermore, the presentinvention provides an arc detection circuit which maintainscompatibility with the welding stations described in U.S. Pat. No.4,716,274.

Therefore, it is an object of the present invention to provide an arcdetection circuit which functions for positive ground, negative ground,and AC output modes of operation.

It is another object of the present invention to provide an arcdetection circuit which, in addition to being usable with positiveground, negative ground, and AC output modes of operation, is alsocompatible with previously existing equipment.

That the present invention accomplishes these objectives and overcomesthe drawbacks of the prior art will be apparent from the detaileddescription of the preferred embodiment below.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an illustration of the preferred environment of the preferredembodiment of the present invention.

FIGS. 2A and 2B are a diagrams of the preferred embodiment of anelectronic welding station 16 which uses the present invention.

FIGS. 3A1, 3A2, 3B1, and 3B2 are diagrams of two embodiments of the basedrive and protection circuit.

FIG. 4 is an illustration of the AC waveforms that are obtained usingthe present invention.

FIG. 5 is a schematic diagram of the high/low voltage switchovercircuit.

FIG. 6 is a schematic diagram of the fan speed control and wire feedspeed control circuits.

FIG. 7A is a schematic diagram of the transistor failure selectioncircuit of the present invention.

FIG. 7B is an illustration of a typical environment in which the presentinvention is used.

FIG. 8 is a graph depicting the pulsewidth and pulse frequency as afunction of the wire feed speed.

FIG. 9A is a schematic of the preferred embodiment of the presentinvention which utilizes the chart of FIG. 8.

FIG. 9B is an alternative embodiment of the present invention using aread-only memory module.

FIG. 10 is an illustration of the core of the inductor/transformer usedin the preferred embodiment.

DETAILED DESCRIPTION

Turn now to the drawing, in which like numerals reference likecomponents throughout the several figures. FIG. 1 is an illustration ofthe preferred environment of the preferred embodiment of the presentinvention. A central welding power supply 10 provides power for a numberof electronic welding stations 16A-16N. The central welding power supply10 may provide an AC output or a DC output. The electronic weldingstations 16A-16N will operate from both AC and DC power input voltages.Central power supply 10 is connected to a source of three-phase AC power(not shown), normally having a voltage of either 230 or 460 volts, byconductors 5, 6, and 7. In the preferred embodiment, the central powersupply 10 provides an output of approximately 80 volts DC at a currentsufficient to power the desired number of electronic welding stations16. Methods of construction of central power supply 10 are well known tothose skilled in the art. Electronic welding stations 16A-16N areconnected by conductors 14A-14N and 15A-15N to central power supply 10.Each electronic welding station 16 can accept an input voltage of 30 to150 volts (80 volts nominal) and has its own controls for varying thearc characteristics and wire feed speed. This allows a welding operatorusing an electronic welding station 16A to adjust the arccharacteristics to match the type of welding that is being performedwithout affecting the arc characteristics of the other electronicwelding stations 16B-16N.

FIG. 2 is a diagram of the preferred embodiment of an electronic weldingstation 16 which uses the present invention. The present inventionrepresents an improvement to electronic welding stations in general and,in particular, to the electronic welding stations described in U.S. Pat.No. 4,716,274, issued Dec. 29, 1987, entitled "Distributed StationWelding System", U.S. patent application Ser. No. 062,543, filed Jun.12, 1987, entitled "Improved Arc Welding System", and U.S. patentapplication Ser. No. 181,985, filed Apr. 15, 1988, entitled "Arc WelderWith Improved Arc Striking Capability", the inventor of all of which isMalcolm T. Gilliland, and all of which are hereby incorporated herein byreference as if fully set forth herein.

VIN conductor 20 and VRET conductor 33 are preferably connected to thepositive and negative outputs, respectively, of the central DC powersupply 10 which powers one or more of the electronic welding stations.Although it is preferred that central power supply 10 have a DC poweroutput, it is permissible for central power supply 10 to have an ACpower output since diodes 21 and 34 provide halfwave rectification. Ofcourse, if the output of central power supply 10 is an AC power outputthen it may be necessary to make capacitor 32 larger than that whichwould be required if central power supply 10 had a DC power output.

VIN conductor 20 is connected to the anode of diode 21. The cathode ofdiode 21 is connected by positive bus 22 to one end of a bleederresistor 31, the positive terminal of filter capacitor 32, one pole ofswitch 26B, the pole of switch 25A, the normally open contact of switch25D, one end of a 100 ohm resistor 102, the cathode of diode 101, and,through switch 26B and conductor 38, to the positive power input oflogic circuits 40. Switch 25 is an eight-pole, double-throw switch whichallows the user to place the electronic welding station 16 in either apositive ground or a negative ground mode of operation. Switch 25(switch sections 25A-25H) is shown in the positive ground position. Forconvenience, switch contacts will be referred to as normally open ornormally closed with respect to the positive ground switch position. Thenormally closed contact of switch 25A is connected through circuitbreaker 26A to positive output bus 27. Circuit breaker 26A and switch26B are embodied in a double pole circuit breaker, or a circuit breakerwith an auxiliary contact (switch). Circuit breaker 26A should be ratedat approximately 100 amps. Positive output bus 27 is connected to thepositive output terminal 28 of the standard output of the electronicwelding station 16, the normally closed contact of switch 25E, thenormally open contact of switch 25G, and the normally closed contact ofswitch 66E.

VRET conductor 33 is connected to the negative power output of centralpower supply 10 and the cathode of diode 34. The anode of diode 34 isconnected by negative bus 35 to the other end of resistor 31, the otherend of capacitor 32, the pole of switch 25B, the normally closed contactof switch 25C and, through switch 36B and conductor 39, to the negativepower input of logic circuits 40. The normally open contact of switch25B is connected to negative output bus 37 through circuit breaker 36A.Negative output bus 37 is connected to negative output terminal 29 ofthe standard output of the electronic welder. Circuit breaker 36A andswitch 36B are also embodied in a double pole circuit breaker. Negativeoutput bus 37 is also connected to the normally open contact of switch25F, the normally closed contact of switch 25G, and the normally closedcontact of switch 66D.

Circuit breakers 26 and 36 operate to shut down the welding station,including the logic circuits, in the event of an overcurrent conditionor an incorrect connection of the welding station to the central powersupply. This helps to limit the degree of any damage.

Consider now the operation of the circuit described thus far. Withswitch 25 in the position shown, which configures the electronic welderfor positive ground operation, positive output terminal 28 is connecteddirectly to positive output bus 22. Negative output terminal 29 isconnected to the negative output bus 35 through the switchingtransistors 57 and 64, output inductors 67 and 75, and output resistors76 and 77. Therefore, in the positive ground configuration, the negativeoutput terminal is the switched terminal and the positive outputterminal is the reference terminal. However, if switch 25 is thrown toits opposite position, which configures the electronic welding stationfor negative ground operation, negative output terminal 29 will bedirectly connected to negative output bus 35 and positive outputterminal 28 will be connected to positive output bus 22 throughswitching transistors 57 and 64, output inductors 67 and 75, and outputresistors 76 and 77. Therefore, in the negative ground configuration,the positive output terminal is the switched output terminal and thenegative output terminal is the reference terminal.

In positive ground operation, positive output terminal 28 is connectedto the workpiece (not shown) and negative output terminal 29 isconnected to the welding torch (not shown). Conversely, in negativeground operation, positive output terminal 28 is connected to thewelding torch and negative output terminal 29 is connected to theworkpiece. In either situation, it will be noted that the workpiece isconnected to a terminal which has direct access to a power supply bus(22 or 35) and the welding torch is connected to the other bus (35 or22) through the switching transistors 57 and 64. Therefore, regardlessof whether positive ground or negative ground operation is selected, thewelding torch is always connected to the switched output of the weldingstation. This allows a multiplicity of units, all configured for eitherthe positive ground mode or the negative ground mode, to operate from acentral power supply 10 and on a common workpiece without the operationof one welding station affecting the operation of another weldingstation through the conductive path provided by the workpiece.

From the above and an inspection of FIG. 2, it will be appreciated thattransistors 57 and 64 are used in a common emitter (inverting)configuration for positive ground operation and in an emitter follower(non-inverting) configuration for negative ground operation. This arisesfrom the use of NPN transistors and the opposite situation would occurwith PNP transistors. It will be appreciated that N-channel andP-channel FETs may be used instead of bipolar transistors.

Parameter selection and logic circuits 40 comprise the circuits forinput voltage sensing, pulse frequency and width selection, chopperfrequency and duty selection, arc detection, output current sensing,pulsewidth modulation, peak current and overcurrent detection, shortcircuit detection, isolation, etc., all as described in the above patentand the above patent applications. The output of logic circuits 40 isconnected by conductors 49, 50 and 51 to the input of base drive andprotection circuit 52. Circuit 52 has isolation circuits, drivers,crowbar circuits and fuses, etc., again as described in the above patentand the above patent applications, and also contains additionalcircuitry so as to provide two outputs. The first output is connected tothe base of transistor 57. The emitter of transistor 57 is connectedthrough resistor 56 to common bus 55. Transistor 57 and resistor 56represent a plurality of parallel connected transistor and resistorpairs, such as shown in FIG. 7 and in the above patent and the abovepatent applications. However, for simplicity of illustration, a singletransistor and a single resistor are shown. The collector of transistor57 is connected to the cathode of diode 60. The anode of diode 60 isconnected by conductor 30 to the normally closed contacts of switches66B and 66C, one end of a snubber 58, one end of a shunt resistor 23,and one end of the primary winding of a pulse transformer 100. The otherend of the primary winding of transformer 100 and the other end ofresistor 23 are connected to the pole of switch 25D.

The second output of circuit 52 is connected to the base of transistor64. The emitter of transistor 64 is connected to emitter bus 55 throughan emitter resistor 63. The collector of transistor 64 is connected tothe cathode of diode 68. The anode of diode 68 is connected to one endof snubber 59 and the pole of switch 66C. The normally closed contact ofswitch 66C is connected to conductor 30. The normally open contact ofswitch 66C is connected by conductor 65 to one end of primary winding67B and the anode of freewheeling diode 70. The other end of winding 67Bis connected to the pole of switch 25G. Transistor 64 and resistor 63also represent a plurality of parallel connected transistor and resistorpairs. Again, for simplicity of illustration, a single transistor and asingle resistor are shown.

In the preferred embodiment, nine transistors/resistor pairs are used:six for transistor 57 and resistor 56, and three for transistor 64 andresistor 63. Also, snubber 58 represents two parallel circuits andsnubber 59 represents one circuit. Each snubber circuit is aconventional snubber circuit and comprises a parallel circuit of aresistor and a diode, connected in series with a capacitor. Transistors57 and 64 and snubbers 58 and 59 are connected in parallel for positiveground and negative ground operation. However, for AC output operation,transistors 57 and 64 are connected in a push-pull arrangement withtransformer 67. The use of six transistors for transistor 57 and threetransistors for transistor 64 is somewhat arbitrary. In the preferredembodiment, each snubber circuit was designed to protect threetransistors so the use of two snubbers and six transistors in one branchand the use of one snubber and three transistors in another branch isfor convenience. It has been found that the use of two transistors fortransistor 64 is quite satisfactory.

The pole of switch 66B is connected through shunt resistor 61 to emitterbus 55. Resistor 61 serves as an arc sustaining resistor in the DCoutput modes during the period when switching transistors 57 and 64 areturned off. Emitter bus 55 is connected to the other end of snubbers 58and 59, the common reference of base drive and protection circuit 52,and the pole of switch 54A. The normally open contact of switch 54A isconnected to the pole of switch 25C. Switch 54 is representative of thecombination of a trigger switch, generally on the torch, and a highcurrent relay, such as are illustrated in the above patent and the abovepatent applications but which, for simplicity, are illustrated here assimply a switch 54. The normally closed contact of switch 25D isconnected by conductor 53 to the normally open contact of switch 25C,the anode of freewheeling diode 73, the cathode of freewheeling diode72, one end of load resistor 74, and one end of output inductor 75. Thecathode of diode 73 and the other end of resistor 74 are connected byconductor 71 to the pole of switch 25E and to the cathode offreewheeling diode 70. The anode of diode 72 is connected to the pole ofswitch 25F. The other end of output inductor 75 is connected to primarywinding 67A of inductor/transformer 67. The other end of winding 67A isconnected to the pole of switch 25G through the series combination ofthe first output resistor 76 and the second output resistor 77.

Freewheeling diode 70 protects transistors 57 and 64 in the positiveground mode of operation. Freewheeling diode 72 protects transistors 57and 64 in the negative ground mode of operation. In the AC output modeof operation, freewheeling diode 70 protects transistor 57 andfreewheeling diode 73 protects transistor 64.

When the electronic welding station is configured, as shown, forpositive ground operation, then any current flowing into negative outputterminal 29 passes through the series combination of switch 25G,resistors 76 and 77, output inductor/transformer 67 and output inductor75, the parallel combination of shunt resistor 23 and pulse transformer100, switching transistors 57 and 64, trigger switch 54, and switch 25Cto negative bus 35. Therefore, negative output terminal 29 is theswitched output terminal. Positive output terminal 28 is connected topositive bus 22 through switch 25A. Because the positive output terminal28 is directly connected to the positive input voltage bus 20, thepositive output terminals 28 of a plurality of welding stations 16 canbe connected to the same workpiece without interaction. In this setup,the torch for a welding station is connected to the negative outputterminal 29 and the workpiece is connected to the positive outputterminal 28 of that welding station.

When the electronic welding station is configured for negative groundoperation current will flow through positive bus 22, the parallelcombination of shunt resistor 23 and pulse transformer 100, transistors57 and 64, trigger switch 54, output inductor 75, outputinductor/transformer 67, output resistors 76 and 77, and switch 25G topositive output terminal 28. Therefore, positive output terminal 28 isthe switched output terminal. Negative output terminal 29 is connectedto negative bus 35 through switch 25B. Because the negative outputterminal 29 is directly connected to the negative input voltage bus 33,the negative output terminals 29 of a plurality of welding station 16can be connected to the same workpiece without interaction. In thissetup, the torch for a welding station is connected to the positiveoutput terminal 28 and the workpiece is connected to the negative outputterminal 29 of that welding station.

The poles of switches 66D and 66E are connected by conductors 48 and 49,respectively, to the AC inputs of a fullwave bridge rectifier 41. Thepositive and negative outputs of bridge 41 are connected by conductors42 and 43, respectively, to the short circuit detector and the high/lowvoltage switchover circuit inputs of logic circuits 40. The basicoperation and function of the short circuit detector and the high/lowvoltage switchover circuit in the logic circuits 40 is the same as thatdescribed in the above patent and patent applications. Briefly stated,the short circuit detector removes the drive signal from transistors 57and 64 in the event the welding rod touches the workpiece, and thehigh/low voltage switchover circuit monitors the output voltage andadjusts the pulse and chopping frequencies and widths, the wire feedspeed, and other parameters so as to promote the striking of the arc.However, a fullwave bridge rectifier 41 has been added so that the shortcircuit detector and the high/low voltage switchover circuit willfunction in the positive, negative and AC output modes. Switch 66controls the inputs to rectifier 41 so that the short circuit detectormay monitor either the standard outputs on terminals 28 and 29 or thealuminum-TIG outputs on terminals 93 and 94.

Resistor 76 has two tap points which are connected by conductors 80 and81 to the inputs of a fullwave bridge rectifier 44. The positive andnegative outputs of rectifier 44 are connected by conductors 45 and 46,respectively, to the inputs of the peak and overcurrent detectors inlogic circuits 41. The basic design and function of the peak andovercurrent detectors in logic circuits 40 are the same as thatdescribed for the peak current detector and overcurrent detector in theabove patent and above patent applications. Briefly stated, the peakcurrent detector and the overcurrent detector remove the drive signalfrom transistors 57 and 64 in the event that the peak current or theaverage current, respectively, through transistors 57 and 64 exceeds apredetermined safe amount. The direction of the current flow throughresistor 76 is dependent upon whether positive ground or negative groundoperation is selected. Therefore, fullwave bridge rectifier 44 rectifiesthe output voltage across the two taps of resistor 76 so that the signalprovided to the logic circuits 40 is independent of the direction ofcurrent flow through the output resistor 76.

Pulse transformer 100 is used for STICK-TIG operation. The constructionand operation of the STICK-TIG circuit has been described in the abovepatent and above patent applications. In the above patent and patentapplications the STICK-TIG circuit monitors the current flow through theswitching transistors 57 and 64 by monitoring the voltage developedacross a resistor, such as shunt resistor 23. However, the STICK-TIGcircuit is sensitive to the DC voltage between resistor 23 and negativebus 35, which is a function of whether the welding station is placed inthe positive ground or the negative ground mode of operation. Therefore,transformer 100 is used to block this DC voltage so that the STICK-TIGcircuit of parameter selection and logic circuits 40 is not sensitive tothe mode of operation. Shunt resistor 23 shunts most of the currentaround pulse transformer 100. Also, because shunt resistor 23 is in theoutput current path, shunt resistor 23 also limits the peak current thatcan flow through transistors 57 and 64. One end of the secondary oftransformer 100 is connected to the anode of diode 101. The STICK-TIGcircuit is also sensitive to voltage polarity. So diode 101 serves as ahalfwave rectifier. The other end of the secondary of transformer 100 isconnected to one end of resistor 103. The other ends of resistors 102and 103 are connected by conductors 105 and 106, respectively, to a 1000ohm load resistor 104 and to the STICK-TIG logic inputs.

The present invention also provides an AC output via the aluminum-TIGoutput terminals 93 and 94. Two general types of AC outputs areavailable: a square wave, which may be symmetrical or asymmetrical, fora cleaning/penetration control mode of welding; and a pulsed wave, wherethere is a variable dead time between the occurrences of pulses, for avoltage control mode of welding. When AC operation is desired, coresection 67E is inserted into core 67D. This removes the air gap in core67D and converts inductor 67 into a transformer. Insertion of coresection 67E also affects switch 66. When core section 67E is insertedswitch 66A (FIGS. 3A and 3B) changes the drive to transistor 64, switch66B is opened, which disconnects arc sustaining resistor 61 from thecircuit, and switch 66C connects transistor 64, through transformer 67,to the pole of switch 25G. Insertion of core section 67E also causesswitches 66D and 66E to switch the inputs of the short circuit detectorand the switchover circuit of logic circuits 40 from the standard outputterminals 28 and 29 to the aluminum-TIG output terminals 93 and 94 sothat the short circuit detector may monitor the AC output for a shortcircuit condition and the switchover circuit may monitor the AC outputfor the striking of the arc.

One end of secondary winding 67C is connected to output terminal 94 byconductor 91. The other end of winding 67C is connected to one end ofthe secondary winding of transformer 86. The other end of the secondarywinding of transformer 86 is connected by conductor 92 to outputterminal 93. A variable direct current power supply 87A is connected tothe primary winding of transformer 86. The fourth winding 67F oftransformer 67 is connected to the output of a second variable DC powersupply 87B. The number of turns on saturation control winding 67F andthe current provided by supply 87B determine the degree of saturation ofthe core 67D of transformer 67. In the preferred embodiment, supply 87Bsimply connects winding 67F to buses 22 and 35 through a 100 ohmrheostat. When operating in an AC output mode, such as the aluminum-TIGmode, a jumper 90 is connected between output terminals 28 and 29, andthe torch and workpiece are connected to terminals 93 and 94.Alternatively, switches manually operated or controlled by the insertionand removal of plug 67E can be used, instead of jumper 90, to connectterminals 28 and 29 and could also be used to switch a pair of outputterminals (not shown) between terminals 28 and 29 and terminals 93 and94, so that the torch and workpiece connections are always to the sameterminals.

Assume now that core 67E is inserted and that jumper 90 has beeninstalled. It will be seen that the end of primary winding 67A with thedot is connected, through output inductor 75, switch 25D, and diode 60,to switching transistor 57. One end of primary winding 67B is connected,through switch 66C and diode 68, to switching transistor 64. The otherend of winding 67A is connected to positive conductor 27 through outputresistors 76 and 77, switch 25G, and jumper 90. The end of winding 67Bwith the dot is connected to positive conductor 27 by switch 25G andjumper 90. In this configuration, transformer 67 may be viewed as beingconfigured in a push-pull configuration with transistors 57 and 64providing the driving action for a tapped primary winding comprisingwindings 67A and 67B. In the preferred embodiment primary winding 67Ahas approximately 23 turns, primary winding 67B has approximately 170turns, secondary winding 67C has approximately 20 turns, and saturationcontrol winding 67F has approximately 200 turns. The relative number ofturns on winding 67A and 67B are selected to compensate for the highercurrent flow through transistor 57 with respect to the current flowthrough transistor 64. Output 1 and output 2 of base drive andprotection circuit 52 alternately drive transistors 57 and 64,respectively. In the preferred embodiment, when the AC output mode ofoperation is selected, only one of these outputs may be on at a time,although both outputs may be off at the same time.

In the preferred embodiment, two modes of operation are possible: thecleaning/penetration control mode, which usually has an asymmetrical ACoutput; and the voltage control mode, which has a pulsed AC output. Inasymmetrical AC operation outputs 1 and 2 are driven 180 degrees out ofphase so that when output 1 is on output 2 is off, and vice versa.Because either transistor 57 or transistor 64 will be on at all timesvarying the pulsewidth of one output or the other output is insufficientto control the arc current. The arc current can be controlled bycontrolling the degree of coupling between the primary winding 67A, 67Band the secondary winding 67C. The degree of coupling can be controlledby controlling the degree of saturation of core 67D, which is effectedby controlling the current flowing through saturation control winding67F. If no current is flowing through winding 67F, then the degree ofsaturation will be small or negligible and full coupling will beachieved. However, if supply 87B provides sufficient current to winding67F to saturate core 67D, then the degree of coupling will be small,thereby limiting the arc current that can be provided. Also, transformer86 and variable supply 87A form a variable reactor which is in serieswith secondary winding 67C and is used to control the arc current. Thereactance is controlled by varying the degree of saturation oftransformer 86. If transformer 86 is saturated then its reactance willbe small. However, if no current is flowing through the primary windingof transformer 86 then transformer 86 will be unsaturated and thereactance will be large, thereby limiting the current through thesecondary. Variable supply 87A controls the amount of current flowingthrough the primary of transformer 86, controls the degree of saturationof transformer 86, controls the reactance of transformer 86, andtherefore controls the arc current. Therefore, the arc current may becontrolled by controlling the degree of saturation of core 67D, thedegree of the saturation of the core of transformer 86, or both.

In the voltage control mode, output 1 and output 2 are never on at thesame time but may be off at the same time. Therefore, transistor 57 willbe turned on for a first predetermined duration, transistors 57 and 64will both be off for a second predetermined duration, transistor 64 willbe on for the first predetermined duration, and then transistors 57 and64 will both be off for the second predetermined duration. The arccurrent can therefore be controlled by controlling the on time of thetwo transistors. In the preferred embodiment, the on time for output 1and the on time for output 2 have the same duration and can be variedfrom 0 to 50 percent of a cycle. Therefore, in this mode of operation,variable supply 87A is set for maximum output so that transformer 86 issaturated and provides the least reactance and variable supply 87B isset for minimum output so that core 67D provides maximum coupling.Switches (not shown) can also be used to bypass the secondary oftransformer 86 and disconnect winding 67F so that supplies 87A and 87Bcan be turned off to reduce power consumption. Of course, if desired,variable supplies 87A and 87B can be used in this mode to control thereactance of transformer 86 and the coupling of transformer 67 toprovide further current limiting capability.

It will therefore be appreciated that the electronic welding stationdescribed above provides positive ground and negative ground weldingcapability as well as two modes of AC output welding operation. It willalso be appreciated that the desired mode of operation can be selectedby a switch and a jumper.

In some types of welding operation output inductor 75 and/or outputresistor 77 may be undesired or unrequired. Therefore, if desired,either one or both of these components may be bypassed by the use ofswitches 83 and 84.

Diodes 60 and 68 prevent circulating currents from heating transistors57 and 64 and from adversely affecting the arc characteristics. Assume,for example, that for a particular condition the base of transistor 57was positive with respect to the collector of transistor 57. If diode 60were not in the circuit then current would flow from output 1, throughthe base-collector junction of transistor 57, through switch 25D, diode73, switches 25E and 25A, resistor 31 and logic circuits 40, andswitches 25C and 54A back into the common terminal of base drive andprotection circuit 52. However, diode 60 prevents this circulatingcurrent from occurring. Similarly, diode 68 prevents a circulatingcurrent from occurring with respect to transistor 64.

The short circuit detector, which is part of logic circuits 40, isdescribed in detail in the above patent and patent applications. Brieflystated, the short circuit detector monitors the outputs for the presenceof a voltage which is at least as great as a predetermined value. If thetorch is shorted to the workpiece, the output voltage will drop to zeroand the short circuit detector will remove the drive signal from basedrive and protection circuit 52, thereby turning off transistors 57 and64 and shutting down the electronic welding station until the short hasbeen removed. In positive ground and negative ground DC outputoperation, when switch 54 is closed, shunt resistor 61 provides aninitial current path so that a DC output voltage appears on the standardoutput terminals 28 and 29 if they are not shorted together. If asufficient voltage is present at the output terminals, the short circuitdetector of the logic circuits 40 is disarmed and allows a drive signalto be provided to the base drive from protection circuit 52 and then totransistor 57. Therefore, if output terminals 28 and 29 are not shorted,shunt resistor 61 will provide an output voltage even if transistors 57and 64 are turned off.

In the AC output modes, such as for aluminum-TIG welding, the shortcircuit detector monitors the aluminum-TIG output terminals 93 and 94.It would seem that the circuit could never turn on because as long astransistors 57 and 64 are turned off there can be no output voltage atterminals 93 and 94 and as long as there is no output voltage atterminals 93 and 94, then the short circuit detector will preventtransistors 57 and 64 from being turned on. However, in thecleaning/penetration control mode of operation, transistor 64 is on whentransistor 57 is off. Therefore, when switch 54 is closed, base driveand protection circuit 52 will turn off transistor 57, but will turn ontransistor 64. This causes a momentary pulse of current to be providedto transformer 67 so that, if terminals 93 and 94 are not shorted, therewill be an output voltage pulse on terminals 93 and 94. This outputvoltage pulse is sufficient to cause the short circuit detector to bedisarmed and allow a drive signal to be provided to the base drive andprotection circuit 52, which then proceeds to drive transistors 57 and64 in a normal manner.

In the voltage control mode of operation, because transistors 57 and 64are not driven in a true 180° phase relationship, removing the drivesignal on conductors 50 and 51 will cause both transistors 57 and 64 tobe turned off. Therefore, transistors 57 and 64 are not able to providea current pulse to disarm the short circuit detector. Also, in thismode, resistor 61 is disconnected by switch 66B and cannot disarm theshort circuit detector. It has been found that, in this mode ofoperation, snubber circuits 58 and 59, even though counteracting eachother's effects somewhat, provide the necessary current pulse totransformer 67 to disarm the short circuit detector. More specifically,when switch 54 is closed a current pulse will flow through snubber 58into primary winding 67A and another current pulse will flow throughsnubber 59 into primary winding 67B. These current pulses do oppose eachother somewhat, but they do not entirely cancel out so a voltage pulseappears on secondary winding 67C. This voltage pulse is sufficient todisarm the short circuit detector in the logic circuits 40. Therefore,in the present invention, in addition to their conventional function,snubber circuits 58 and 59 also provide a starting pulse which disarmsthe short circuit detector and enables the electronic welding circuit.Of course, if the rod is touched to the workpiece such that terminals 93and 94 are shorted then the current pulse will be unable to generate asufficient voltage to disarm the short circuit detector and theelectronic welding station will remain inoperative until the short isremoved.

Even when plug 67E is removed from core 67D there will be some couplingbetween windings 67A, 67B and 67C. If winding 67C sees an open circuitthen the voltage at terminals 93 and 94 can provide a shock and presenta safety hazard. Therefore, a 500 ohm resistor 62 is connected acrossterminals 93 and 94 and presents a load to winding 67C. This loadprevents the voltage across terminals 93 and 94 from being a safetyhazard in the positive ground and negative ground modes of operation,and is an inconsequential load in the AC output modes of operation.

Turn now to FIG. 3A which is a diagram of one embodiment of base driveand protection circuit 52. In the preferred embodiment, logic circuits40 contain a pulsewidth modulator, such as the LM3524 manufactured byNational Semiconductor Products, Inc., Santa Clara, Calif. Theprincipals of operation of logic circuits 40 have been described indetail in the above patents and the above patent applications and,therefore, only the modifications thereto will be discussed.

A first emitter output EA for modulator 120 is connected by conductor 50to the normally open contact of switch 121A, one end of resistor 115,and the anode of the diode of an optoisolator 116, such as the M57215L,manufactured by Mitsubishi Electric Company, Tokyo, Japan. A secondemitter output EB is connected by conductor 51 to the pole of switch121A, one end of resistor 124, and one end of resistor 123. The otherend of resistor 124 is connected to the anode of the diode of a secondoptoisolator 125, such as the MCT-2, manufactured by Motorola, Inc.,Phoenix, Ariz. The ground terminal GND of modulator 120 is connected byconductor 49 to the cathodes of the diodes of optoisolators 116 and 125and to the other ends of resistors 115 and 123. The frequency ofoscillation of modulator 120 is controlled by the resistance presentbetween the terminal RT and the ground terminal GND. Terminal RT isconnected to one end of potentiometer 126A and potentiometer 126B. Thewiper and other end of potentiometer 126A are connected to the normallyclosed contact of switch 121B. The wiper and other end of potentiometer126B and the pole of switch 121B are connected by conductor 49 to theground terminal. In the preferred embodiment, potentiometers 126A and126B have the same value and are ganged.

The transistor of optoisolator 116 is connected to the input of the maintransistor bank drive circuit 117. The output of circuit 117 isconnected to the input of main transistor bank protection circuit 118.The output of the main transistor bank protection circuit 118 isconnected to the base of transistor 57. Transistor 57 is connected aspreviously described in conjunction with FIG. 2.

The collector of the transistor of optoisolator 125 is connected throughresistor 127 and conductor 130 to the VS power input of driver 133 andthe +12 volt output of isolated power supply 140. The emitter of thetransistor of optoisolator 125 is connected to one end of a pull downresistor 135, the cathode of a 4.7 volt zener diode 134, and the ENABLEinput of driver 133. The ground terminal GND and the PULSE input ofdriver 133 are connected by conductor 132 to the anode of diode 134, theother end of resistor 135, and the -12 volt output of power supply 140.In the preferred embodiment, driver 133 is the SG3635P half bridgedriver manufactured by Silicon General, Garden Grove, Calif. The outputof driver 133 is connected by conductor 137 to one end of a loadresistor 136 and the pole of switch section 121D. The other end ofresistor 136 is connected by conductor 131 to the pole of switch section121C and to the common terminal of power supply 140. The normally closedcontact of switch section 121D and normally open contact of switchsection 121C are connected by conductor 147 to the source and substrateof an N-channel insulated gate FET and the source and substrate of aP-channel insulated gate FET 150, the node of diode 146, the cathode ofdiode 151, and the normally open contact of switch section 66A. Switchsection 66A is shown in the normal (DC) position. The drain oftransistor 150 and the anode of diode 151 are connected by conductor 142to the -6 volt output of isolated power supply 141. The drain oftransistor 145 and the cathode of diode 146 are connected throughcurrent limiting resistor 144 and conductor 143 to the +6 volt output ofpower supply 141. The common terminal of power supply 141 is connectedto emitter bus 55. The pole of switch 66A is connected to the base oftransistor 64 through auxiliary transistor bank protection circuit 119.

The normally closed contact of switch section 66A is connected to theoutput of main transistor bank drive circuit 117. When the weldingstation is in the positive ground or the negative ground operating mode,transistor 64 is driven by drive circuit 117. In the AC output modes ofoperation, transistor 64 is driven by FETs 145 and 150.

Assume now that switch 66 is moved to select the AC output mode ofoperation. In the configuration shown, driver 133 functions as aninverter. That is, when the ENABLE input is high, the output is low (-12volts). However, with switch 121 in the position shown, the output isconnected to the sources of transistors 145 and 150 and the commonterminal of power supply 140 is connected to the gates of transistors145 and 150. Therefore, when the ENABLE input is high transistor 145 isturned on and transistor 150 is turned off, the net result of which isto apply a positive voltage to the base of auxiliary transistor 64 andturn on transistor 64.

If switch 121 is in the other (cleaning/penetration control mode)position, then the output of driver 133 will be connected to the gatesof transistors 145 and 150. In this case, when the ENABLE input is hightransistor 145 will be turned off and transistor 150 will be turned on,thereby applying a negative voltage to the base of transistor 64 andturning it off.

Construction of isolated power supplies 140 and 141 is well known tothose of ordinary skill in the art. Furthermore, the above U.S. patentand patent applications disclose the construction of isolated powersupplies for use in an electronic welder and the construction of crowbarand fuse protection circuits such as circuits 118 and 119.

Consider now the effects of switch 121. Switch 121 has two positions: avoltage control mode position, which provides a pulsed output, and acleaning/penetration control mode position, which provides a symmetricalor an asymmetrical rectangular waveform output. Switch 121 is shown inthe voltage control mode position. Outputs EA and EB have three possibleoperating modes: EA on and EB off; EA off and EB on; and both EA and EBoff. When switch 121 is in the position shown, the voltage control modeposition, output EA drives optoisolator 116, output EB drivesoptoisolator 125, and potentiometers 126A and 126B are connected inparallel. If outputs EA and EB are connected in parallel, such as whenswitch 121 is in the cleaning/penetration control mode position, theoutput frequency at this paralleled output is equal to the oscillationfrequency. However, if outputs EA and EB are used independently, theneach output has a frequency of one-half of the oscillation frequency.When switch 121 is in the voltage control mode position, the outputs areindependent but, because of the insertion of potentiometer 126A into theoscillation frequency circuit, the oscillation frequency is doubled and,therefore, the output frequency of each output is the same as theoriginal output frequency. Outputs EA and EB have the same frequency andpulsewidth, but are displaced from each by one oscillation cycle ofmodulator 120. Therefore, modulator 120 drives optoisolator 116 for afirst period having a duration of T1, provides no output for a secondperiod having a duration of T2, drives optoisolator 125 for a thirdperiod having a duration of T1 and, to complete the cycle, provides nooutput for a fourth period having a duration of T2. The result is thattransistor bank 57 is turned on for a duration T1, there is a dead timeof T2, transistor bank 64 is turned for a duration of T1, and then thereis another dead time T2.

Consider now the effect of placing switch 121 in the opposite(cleaning/penetration control mode) position. Outputs EA and EB areconnected in parallel and simultaneously drive optoisolators 116 and125. Also, potentiometer 126A is removed from the oscillation circuit ofmodulator 120 so that the oscillation frequency is determined only bypotentiometer 126B. Switch sections 126C and 126D reverse the driveinputs to the FETs 145 and 150 so that, in effect, the signal going intothe FETs is inverted. Now, whenever output EA or output EB is on,optoisolator 116 and main transistor bank drive circuit 117 will turn onthe main transistor bank 57 and FET 150 will turn off auxiliarytransistor bank 64. However, when output EA and output EB are bothturned off, then optoisolator 116 and drive circuit 117 will turn offmain transistor bank 57, but optoisolator 125, driver 133, and FET 145will turn on auxiliary transistor bank 64. Therefore, in thecleaning/penetration control mode of operation, either main transistorbank 57 or auxiliary transistor bank 64 is turned on.

Turn now to FIG. 3B which is a diagram of another embodiment of basedrive and protection circuit 52. This circuit is similar to the circuitof FIG. 3A except that transistor 64 is always driven by FETs 145 and150 so that all of the drive components are used in both the DC outputmodes and the AC output modes of operation. The difference is betweenthe circuit of FIG. 3B and the circuit of FIG. 3A are as follows. Switch66F, which is part of switch 66, is connected in parallel with switch121A. In the DC modes of operation switch 66F connects outputs EA and EBtogether. In the AC modes of operation, switch 66F presents an opencircuit. Switch 66G is connected in a series with switch 121B. In the ACmodes of operation, switch 66G is closed and, in the DC modes ofoperation, switch 66G is open. The output of driver 133 is connected byconductor 137 to the pole of switch 66I and one end of resistor 136. ForAC output operation, switch 66I connects conductor 137 to the pole ofswitch 121D. For DC output operation, switch 66I connects conductor 137to conductor 147, thereby overriding the selection of switch 121D. Theend of the resistor 136 is connected to conductor 131 and the pole ofswitch 66H. In the AC output modes of operation, switch 66H connectsconductor 131 to the pole of switch 121C. In the DC modes of operation,switch 66H connects conductor 131 to conductor 148, thereby overridingswitch 121C. In the AC output modes of operation, the circuit of FIG. 3Bfunctions exactly as the circuit of FIG. 3A. In the DC modes ofoperation, switch 66F connects outputs EA and EB of modulator 120 inparallel, switch 66G prevents switch 121B from altering the pulsefrequency, and switch sections 66H and 66I connect the output of drive133 to the inputs of FETs 145 and 150 so that auxiliary transistor bank64 is always driven by the combination of optoisolator 125, driver 133,and FETs 145 and 150.

Turn now to FIG. 4 which is an illustration of the AC waveforms that areobtained using the present invention. Outputs EA and EB are shown aswaveforms 170 and 171, respectively. For clarity of illustration, theseoutputs are shown as being independent even though switch 121 ties theoutputs together in the cleaning/penetration control mode position.Output EA is shown as a series of narrow pulses, having a pulsewidth ofT1, from time TA to time TB, and then as a series of wider pulses havinga pulsewidth of T1'. Likewise, output EB is also shown as a series ofnarrow pulses having a pulsewidth T1 from time TA to time TB, and then aseries of wider pulses having a pulsewidth of T1'. The dead time, thetime when neither output EA nor output EB is active, has a duration ofT2 between times TA and TB, and a smaller duration T2' from then on. Itwill be noticed that outputs EA and EB are offset from each other by aduration (T1+T2) which is equal to one-half of the period of thefrequency of oscillation of outputs EA and EB.

It will be recalled from the discussion of FIG. 3 that, in the voltagecontrol mode of operation, output EA drives the main transistor bank 57and output EB drives the auxiliary transistor 64, and that thesetransistors drive, in a push-pull configuration, transformer 67. Theoutput at terminals 93 and 94, in the voltage control mode, is shown aswaveform 172. It will be noted that the positive and negative pulseshave the same duration and that a variable dead time is present betweenthe pulses.

It will also be recalled that, in the cleaning/penetration control mode,outputs EA and EB are connected in parallel and drive main transistorbank 57 and auxiliary transistor bank 64 in a 180° phase relationship.The output of transformer 67 is shown as waveform 173. It will be notedthat the pulsewidth of one polarity of output voltage need not be thesame as the pulsewidth of the other polarity of output voltage, and thatthe output is either positive or negative so that there is no dead time,such as the dead time shown in waveform 172.

For convenience, outputs 172 and 173 are shown as if transformer 67could pass a DC signal. However, it will be appreciated that transformer67 cannot pass DC and the positive polarity/negative polarity crossoverpoint on these waveforms will vary as a function of the mode ofoperation, the pulsewidth, the arc current, and the particularcharacteristics of the welding operation being performed.

Turn now to FIG. 5, which is a schematic diagram of the high/low voltageswitchover circuit. The construction and operation of this circuit isessentially the same as the high/low voltage switchover circuitdescribed in the above patents and patent applications. Some changeshave been made to the circuit to accommodate the selectable outputpolarity feature of the present invention. Conductor 38, which isnominally at +80 volts with respect to conductor 39, is connected to theinput of a 15 volt regulator 200 and the pole of relay 211. The outputof regulator 200 is connected by conductor 201 to the positive terminalof capacitor 202, one end of capacitor 203, one end of resistor 205, andthe positive power input of a comparator or operational amplifier 204.The other end of resistor 205 is connected by conductor 207 to theinverting input of comparator 204 and the cathode of an 8.2 volt zenerdiode 206. The output of comparator 204 is connected by conductor 209 tothe cathode of diode 210 and one end of the coil of relay 211. The anodeof diode 210 and the other end of the coil of relay 211 are connected tothe anode of diode 212. Conductor 39 is connected to the ground terminalof regulator 200, the other ends of capacitors 202 and 203, the anode ofzener diode 206, the cathode of diode 212, and the negative power inputof comparator 209. The positive output terminal 28 is connected by aremovable jumper 240 to the anode of diode 220. The cathode of diode 220is connected through resistor 221 and conductor 228 to the non-invertinginput of comparator 204, one end of a 0.1 microfarad capacitor 222, oneend of a 27 k resistor 223, one end and the wiper of a 250 kpotentiometer 224, and the cathode of diode 225. The other end ofcapacitor 222, resistor 223, and potentiometer 224 are connected toconductor 39.

The positive output of bridge 41 (FIG. 2) is connected by conductor 42to one end of a 20 k potentiometer 232, the cathode of a 12 volt zenerdiode 233, and the positive input terminal of the amplifier 227A of anisolation amplifier 227, such as the type AD210 manufactured by AnalogDevices, Norwood, MA. The negative output of bridge 41 (FIG. 2) isconnected by conductor 43 to the cathode of diode 230. The anode ofdiode 230 is connected through an 82 k current limiting resistor 231 toone end of a 20 k potentiometer 232. The wiper of potentiometer 232 isconnected to the anode of zener diode 233 and the reference input ofamplifier 227A. Amplifier 227A is configured as a unity gain amplifier.The output of amplifier 227A is coupled to the input of amplifier 227Cthrough an isolation circuit 227B. The output of amplifier 227C isconnected by conductor 226 to the anode of diode 225. The referenceterminal of amplifier 227C is connected to conductor 39. A 12-14 volt DCpower supply 234 is connected by conductors 235 and 236 to the internalpower supply 227D of isolation amplifier 227.

Regulator 200 provides operating power for comparator 204. Zener diode206 provides a reference voltage for comparator 204. Resistors 221 and223 and potentiometer 224 allow for the adjustment of the signal inputvoltage to comparator 204 so that, before the arc is struck, the voltageat the non-inverting input of comparator 204 is greater than thereference voltage provided by zener diode 206. This causes comparator204 to energize relay 211, which thereby makes the connection betweenconductors 38 and 214. Once the arc is struck, the voltage at terminal28 will drop to a lower voltage and cause the signal input voltage tocomparator 204 to be less than the reference voltage. Comparator 204will then de-energize relay 211 which in turn causes conductors 38 and213 to be connected. Therefore, +80 volts is applied to conductor 214before the arc is struck and, once the arc is struck, the +80 volts isthen switched to conductor 213. Conductors 213 and 214 are connected tocircuits which are required to switch functions depending on whether thearc has or has not yet been struck, or whether the arc is present or isnot present, such as circuits which control the pulse repetitionfrequency, the pulsewidth, the modulating pulse frequency, themodulating pulsewidth, current limiting thresholds, etc. The operationof such circuits is described in the above patent and patentapplications.

When the welding station is in the DC output modes of operation, beforethe arc is struck, positive output terminal 28 will be at approximately+80 volts with respect to negative output terminal 29, the output ofamplifier 227C will be a higher voltage than the reference voltageprovided by zener diode 206, and comparator 204 will energize relay 211.However, once the arc is struck, the voltage difference betweenterminals 28 and 29 will drop to approximately 20 volts, which is thearc sustaining voltage. Therefore, the input voltage to amplifier 227Awill be reduced and the output voltage of amplifier 227C will drop belowthe reference voltage provided by zener diode 206. This will causecomparator 204 to de-energize relay 211. Therefore, regardless ofwhether the welder is operated in the positive ground mode or thenegative ground mode, relay 211 will be energized before the arc isstruck and de-energized after the arc is struck.

The operation of the circuit is the same for the two AC output modes ofoperation but, in these modes, switch sections 66D and 66E switch theinputs for the switchover circuit from terminals 28 and 29 to terminals93 and 94 so that the switchover circuit monitors the AC output voltage.

Calibration of the circuit is as follows. Potentiometer 232 is firstadjusted so that the wiper is connected to conductor 42. This disablesisolation amplifier 227. A jumper 240 is then connected between positiveoutput terminal 28 and the anode of diode 220. Potentiometer 224 is thenadjusted so that relay 211 is energized before the arc has been struckand de-energized once the arc has been struck. Jumper 240 is thendisconnected and potentiometer 232 is adjusted so that relay 211 isenergized before the arc is struck and de-energized after the arc hasbeen struck. It will be appreciated that the output of isolationamplifier 227 can be used to directly drive a buffer amplifier which inturn drives relay 211. In this case, comparator 204 would be replaced bya buffer amplifier and most or all of components 205, 206, 220, 221, and224 could be eliminated. However, the inventor desires that this circuitbe compatible with other electronic welding stations manufactured by theinventor, such as those described in the above patent and patentapplications. Therefore, in the preferred embodiment, all the componentsare placed on a single circuit board and, for welding stations which donot have the reversible polarity or AC outputs, conductors 42 and 43 aredisconnected and the jumper 240 is installed. On electronic weldingstations which have these additional output features, jumper 240 isdisconnected and conductors 42 and 43 are connected.

Turn now to FIG. 6 which is a schematic diagram of the fan speed controland the wire feed speed control circuits. The switched 80 volt line 213from the high/low voltage switchover circuit (FIG. 5) is connected toone end of the coil 250A of transfer relay 250. The other end of coil250A is connected to the fused 80 volt return line 39. After the welderhas pulled the trigger switch, but before the arc has been struck,conductor 213 will not have any voltage applied to it and the coil 250Aof relay 250 will not be energized. However, once the arc has beenstruck, conductor 213 will have 80 volts applied to it and coil 250A ofrelay 250 will be energized. Conductor 22, which is connected to +80volts, is connected to the normally open contact of relay section 250Band to the normally closed contact of switch section 25H. As will berecalled from an inspection of FIG. 2, switch 25 is used to selectpositive ground or negative ground operation. The pole of switch section25H is connected to the normally closed contact of relay section 250B.The pole of relay section 250B is connected by conductor 251 to one endof resistor 252, one end of resistor 257, and the pole of relay section250C. The tap on resistor 252 is connected by conductor 253 to thepositive power input of the wire feeder motor and speed control circuit254. Conductor 35, which is the 80 volt return conductor, is connectedto the other end of resistor 252, fan and reversing circuit 260, and thenegative power input of the wire feeder motor and speed control circuit254. The normally open contact of relay section 250C is connected byconductor 255 to the contact of switch 54B. The pole of switch 54B isconnected by conductor 256 to the other end of resistor 257 and to thefan and reversing circuit 260. Conductor 85 is connected to the normallyopen contact of switch section 25H. It will be recalled from aninspection of FIG. 2 that conductor 85 is connected to the junctionbetween the output inductors/transformers 67 and 75 and the outputresistors 76 and 77.

In the positive ground mode and in the AC modes of operation, after thewelder pulls the trigger, but before the arc is struck, relay 250 willnot be energized and the full 80 volts on conductor 22 will be appliedvia switch section 25H and relay section 250B to resistors 252 and 257.Resistor 252 is tapped so as to provide only 50 to 65 volts, withrespect to conductor 35, to the wire feeder control circuit 254.Resistor 257, which is in series with fan and reversing circuit 260,reduces the voltage available to circuit 260 so as to reduce the fanspeed and prolong the life of the fan brushes. The function of resistor257 and the construction and operation of fan and reversing circuit 260is described in detail in the above patent applications. Once the archas been struck, relay 250 will be energized and the full +80 volts onconductor 22 will be applied to resistor 252, as before, and will alsobe directly applied to the fan and reversing circuit 260 through relaysection 250C and switch section 54B. Therefore, once the arch has beenstruck, the fan of circuit 260 will run at the high speed necessary tocool the electronic welding station and prevent it from overheating.Resistor 252 will again tap down the voltage so as to provide thedesired 50 to 65 volts to wire feed circuit 254.

When negative ground operation is selected, the normally closed contactof relay section 250B will be connected via switch section 25H toconductor 85. Therefore, once the trigger 54 is pulled, but before thearc has been struck, relay 250 will not be energized and the wire feedcircuit 254 and the fan and reversing circuit 260 will be powered by themain transistor bank 57. Resistors 252 and 257 reduce the voltage sothat wire feed circuit 254 and fan and reversing circuit 260,respectively, once again receive the proper operating voltage. However,once the arc has been struck, relay 250 will be energized and wirefeeder circuit 254 and fan and reversing circuit 260 will be powereddirectly from the 80 volt conductor 22. Again, once the arc has beenstruck, the fan of circuit 260 will run at full speed and resistor 252will tap down the voltage for wire feed circuit 254. This circuit, inconjunction with the high/low voltage switchover circuit of FIG. 5,serves to maintain the proper voltage and polarity for the fan andreversing circuit 60 and the wire feeder motor and speed control circuit254, regardless of the mode of operation of the electronic welder.

Turn now to FIG. 7A which is a schematic diagram of the transistorfailure selection circuit of the present invention, and FIG. 7B which isan illustration of a typical environment in which the present inventionis used. Base drive and protection circuit 52 has a plurality ofoutputs: 1A through 1N. Each of these outputs is independently protectedby fuse and crowbar circuits described in the above referenced patentapplications. Outputs 1A through 1N are connected to the bases oftransistors 57A through 57N, respectively, through balancing/currentlimiting resistors 271A through 271N, respectively. The COMMON terminalof base drive and protection circuit 52 is connected through conductor55 and a plurality of emitter resistors 56A through 56N to the emittersof transistors 57A through 57N, respectively. The collectors oftransistors 57A through 57N are connected to the cathode of diode 60 andthe anode of diode 60 is connected to conductor 30. In a typicalenvironment, transistors 57A through 57N will be mounted on a singleheatsink 270. Heatsink 270 and most of transistors 57A through 57N, onceinstalled in the welding station, will typically be very difficult orimpossible to access without unsoldering and/or disconnecting and/orremoving other circuit boards, fans, switches, relays, etc. In thepreferred embodiment of the present invention, at least one of thetransistors 57 is readily accessible for removal and replacement. In thepreferred embodiment, transistor 57N is at the top part of heatsink 270and is readily accessible for removal and replacement. An overloadcondition, short circuit condition, or other condition may occur whichcauses one of the transistors 57 to fail regardless of the presence ofthe protection circuits included in the welding station. The presentinvention is concerned with forcing these conditions to cause thefailure of readily accessible transistor 57N in preference to thefailure of the less readily accessible transistors 57A-57M. It will beunderstood that the present invention is not intended to cause thefailure of transistor 57N, but is concerned with selecting transistor57N as the transistor to fail in the event that circumstances orconditions cause an overload which is going to force one of thetransistors 57 to fail.

In the preferred embodiment, the preferential failure of transistor 57Nis achieved by the use of a higher value for base resistor 271N than thevalue for base resistors 271A through 271M. The higher value for baseresistor 271N limits the base drive for transistor 57N, as compared tothe base drive for transistors 57A through 57M. This will causetransistor 57N to come out of saturation, or near saturation, into theactive operating region before any of transistors 57A through 57M. Thiscauses transistor 57N to heat up more than transistors 57A through 57Mand transistor 57N then fails first. Generally, this overheating willcause transistor 57N to fail by suffering a collector-base junctionshort. When the collector-base junction shorts, the collector voltagewill be fed back through the base of transistor 57N into output 1N ofbase drive and protection circuit 52. This will cause the fuse andcrowbar circuits in base drive and protection circuit 52 to fire andremove base drive from the remaining transistors 57A through 57M. Oncethe base drive is removed from transistors 57A through 57M, theremaining current flow will be via transistor 57N. If the conditionwhich caused the overload has not been corrected, then the excessivecurrent will further damage transistor 57N and cause transistor 57N toeventually appear as an open circuit. Therefore, the judicious selectionof base resistor 271N is used so that a short circuit or other overloadcondition causes the failure of transistor 57N in preference to thefailure of any of the other transistors 57A through 57M.

In an alternative embodiment, the preferential failure of transistor 57Nis achieved by the use of a lower value for emitter resistor 56N thanthe value for emitter resistors 56A through 56M. The lower value foremitter resistor 56N will also cause transistor 57N to come out ofsaturation, or near-saturation, before any of transistors 57A through57M in the event of an overcurrent condition. This also causestransistor 57N to heat up more than transistors 57A through 57M andtransistor 57N then fails first. Therefore, the judicious selection ofemitter resistor 56N is used so that a short circuit or other overloadcondition causes the failure of transistor 57N in preference to thefailure of any of the other transistors 57A-57M.

Although the above discussion has been directed towards transistor 57Nit will be appreciated that the same principles of operation can beapplied to cause the preferential failure of a different transistor,such as transistor 57C, if that transistor is the most readilyaccessible transistor.

Although the above discussion has been directed towards transistor 57N,it will be appreciated that the same principles of operation can beapplied to cause the preferential failure of one of the transistors usedfor transistor 64.

In the preferred embodiment, base resistors 271A through 271M are eachconstructed of six inches of No. 16 wire. Base resistor 271N isconstructed of two 7.5 inch No. 22 wires, connected in parallel. Theresistance of No. 16 copper wire is approximately 4.09 ohms per 1,000feet at 25° C. The resistance of No. 22 copper wire is approximately16.4 ohms per 1,000 feet at 25° C. Base resistors 271A through 271M,therefore, each have a resistance of approximately 0.002 ohms and baseresistor 271N has a resistance of approximately 0.005 ohms. It will beappreciated that other wire sizes and materials can be used as long asthe base resistor for the selected transistor has a higher value thanthe base resistor for any of the other transistors. In the preferredembodiment, the ratio of the high value to the low value isapproximately 2.5, but this ratio is not critical. The materials used inthe preferred embodiment were selected because of their readyavailability and low cost.

In the alternative embodiment, emitter resistors 56A through 56M eachhave a resistance of 0.0125 ohms and emitter resistor 56N has aresistance of 0.01 ohm. In this alternative embodiment, the ratio of theresistance value is approximately 1.25, but this ratio is not critical.

Although the above discussion has been directed towards causing thepreferential failure of one transistor 57, it will be appreciate thatthis technique is not limited to one transistor, but can be used tocause the preferential failure of two or more transistors which arereadily accessible. For example, if transistors 57B and 57N were bothreadily accessible, then base resistors 271B and 271N would have ahigher value than the remaining base resistors and/or emitter resistors56B and 56N would have a lower value than the remaining emitterresistors.

The present invention should also be understood as providing a techniquewhereby less readily accessible transistors are protected in preferenceto more readily accessible transistors. Therefore, if transistors 57Athrough 57M were readily accessible, but transistor 57N was not readilyaccessible then base resistor 271N and/or emitter resistor 56N wouldhave different values than base resistors 271A through 271M and/oremitter resistors 56A through 56M, respectively, so that transistor 57Nwould be the least likely to fail in the event of a short circuit oroverload condition.

Although the use of resistors in the base and/or emitter circuit of eachtransistor is preferred, it will be appreciated that transistorselection may also be accomplished by the use of a resistor in thecollector circuit of each transistor. In this case the use of a lowervalue resistance in the collector circuit of the selected transistorwill result in a higher collector-emitter voltage, and therefore ahigher heat generation, for the selected transistor than for the othertransistors. Likewise, a higher value resistance will result in lessvoltage, less heat generation, and a reduced likelihood of failure.

Also, although the use of bipolar transistors is currently preferredfrom a cost viewpoint, it will be appreciated that field effecttransistors may be used for transistors 57 and 64. In this case, theresistor or resistors would be in the source, drain, and/or gate circuitof each transistor and the resistance value(s) used with the selectedtransistor would be different than the resistance value(s) used with theother transistors.

Turn now to FIG. 8 which is a graph depicting the pulsewidth and pulsefrequency as a function of the wire feed speed for a hypotheticalwelding operation. The preferred embodiment contemplates a singlecontrol which simultaneously varies the pulse frequency, pulsewidth,wire feed speed and/or other parameters. In the method practiced by thepresent invention a graph, such as that depicted by FIG. 8, is made fora particular welding operation. To arrive at the graph for a particularwelding operation, a control parameter is selected, such as the wirefeed speed. A first wire feed speed is then selected and the pulsefrequency and pulsewidth, or other desired dependent parameter, arevaried to achieve the most desirable weld at that wire feed speed. Thewire feed speed, the pulse frequency and the pulsewidth, or otherdesired dependent parameter, are recorded. The wire feed speed is thenchanged and the pulse frequency and pulsewidth, or other desireddependent parameter, are again varied until the most desirable weld isproduced and this information is recorded. This procedure is repeatedthrough the range of wire feed speeds of interest and the recorded datais used to plot a graph of the dependent parameters (pulsewidth, pulsefrequency, etc.) as a function of the independent parameter (the wirefeed speed). Of course, if desired, the wire feed speed, pulsefrequency, and other parameters can be plotted as functions of thepulsewidth or other parameters, the wire feed speed, pulsewidth, andother parameters can be plotted as functions of the pulse frequency orsome other parameter, or one or more of these parameters could beplotted as a function of some other parameter.

After the graph is drawn, it will be often noted that the pulsewidth 281and the pulse frequency 282 are not precise functions nor linearfunctions of the wire feed speed 280. In the example shown, thepulsewidth 281 has one slope up to wire feed speed S2 and a second slopefrom that point on. The pulse frequency 282 has a first slope to speedS1, a second slope from speed S1 to speed S3, a third slope from speedS3 to speed S4, and a fourth slope from speed S4 on. It should be notedthat FIG. 8 is not intended to be a graph of any particular weldingoperation but is intended only to be indicative of the type of graphthat could be plotted for a welding operation. Once the chart has beenmade three custom potentiometers can be manufactured and ganged so thatthe three potentiometers track to produce the charted relationshipbetween the pulsewidth, pulse frequency, and wire feed speed. It will beappreciated that a desired precise and/or non-linear curve can beobtained using, as appropriate, a linear taper or a non-linear taper forthe potentiometer in conjunction with one or more taps, each tap beingconnected to an appropriate voltage through an appropriate resistor.Even though specific reference has been made to certain parameters beingdependent upon the wire feed speed, the present invention contemplatesand includes the concept of one more selected parameters (pulsefrequency, pulsewidth, gas flow rate, etc.) being dependent upon apreselected independent parameter (wire feed speed, torch travel rate,etc.).

Although the chart depicted in FIG. 8 and the embodiments depicted inFIGS. 9A and 9B indicate that two parameters, such as the pulsewidth andthe pulse frequency, will be dependent upon a control parameter, such asthe wire feed speed, it will be appreciated that circumstances may occurin which only one parameter is dependent upon the control parameter, ormore than two parameters are dependent upon the control parameter.

This technique is not limited to pulse arc welding machines but is alsoapplicable to other types of welding machines, such as a standard MIGwelding machine operating in a high density spray mode wherein only thewire feed speed and the output voltage are controlled. Of course, itwill be appreciated that the output voltage is frequently a function ofthe pulsewidth and/or the pulse frequency.

Turn now to FIG. 9A which is a schematic of the preferred embodiment ofthe present invention which utilizes the chart of FIG. 8. An electronicwelding station 300 contains a power, control and logic circuit 301, apulsewidth modulator 302, and a switching circuit 303, interconnected bya signal and control bus 310, which carries signals controlling theoperation of the electronic welding station 300. A power and control bus304 connects power, control and logic circuit 301 to connector 307.Pulsewidth modulator 302 has a frequency control input and a pulsewidthcontrol input, which are connected by buses 305 and 306, respectively,to connector 307. The DC output of switching circuit 303 is connected byconductors 27 and 37 to output terminals 28 and 29, respectively, andthe AC output of switching circuit 303 is connected by conductors 92 and91 to output terminals 93 and 94, respectively.

The welding torch and feeder assembly 320 represents the wire feeder andthe welding gun and cable assembly (a torch), or a torch with a built-inwire feed motor, etc. Typically, welding torch and feeder assembly 320will comprise a wire feeder attached to a torch.

The welding torch and feeder assembly 320 contains a wire feed speedcontrol circuit 323 and a motor and wire feed mechanism 325 for feedingwelding rod 331. The power, braking, and control inputs of controlcircuit 323 are connected by bus 322 to connector 321. Bus 322 alsoconnects connector 321 to connector 330. The output of control circuit323 is connected by bus 324 to the motor power inputs of wire feedmechanism 325. The arc power input of mechanism 325 is connected byconductor 326 to a selected one of output terminals 28, 29, 93 or 94 ofelectronic welding station 300. The workpiece is connected by conductor311 to another appropriate output terminal. The speed control input ofcontrol circuit 323 is connected by bus 327 to connector 330. In thepreferred embodiment, welding torch and feeder assembly 320 has anopening or cavity 335 into which a control module 340 can be inserted.Because of size limitations, module 340 would typically be inserted intoa cavity in the wire feeder, as opposed to the torch. However, if module340 were small enough, then it could be inserted into a cavity on thetorch.

Control module 340 preferably has a control knob 342, a three-gangpotentiometer 341, and a connector 343. When module 340 is inserted intocavity 335 such that connectors 330 and 343 are mated, speed controlpotentiometer 341A will be connected to the speed control input ofcontrol circuit 323 and frequency and pulsewidth potentiometers 341B and341C, respectively, will be connected to the frequency and pulsewidthcontrol inputs, respectively, of pulsewidth modulator 302. Turning knob342 simultaneously adjusts the wire feed speed, the pulse frequency, andthe pulsewidth so as to produce a quality weld at the highest possiblewire feed speed. This approach allows the person doing the welding toadjust the wire feed speed to match his own capabilities, which may varyfrom day to day and even within a day, without having to stop andreadjust the pulse frequency and pulsewidth to produce the desired weld.The efficiency and speed of the welding operator are thus improved andthe quality of the weld is less susceptible to misadjustment of controlsby the operator.

It is preferred that control knob 342 and potentiometers 341A-341C bepart of a replaceable module 340. This allows one module to be used fora first type of welding operation, such as where the workpiece is steel,and another module, customized for aluminum, to be used when theworkpiece is aluminum plate. For a given project, the number ofdifferent types of welding operations is usually very limited so that arelatively small number of replaceable modules 340 will provide thewelding operator with an electronic welding station whosecharacteristics are tailored to the particular welding operation beingperformed. It is preferred that the module 340 be inserted into thewelding torch and feeder assembly 320 so that the welding operator doesnot have to interrupt the welding operation and leave the work in orderto make an adjustment to the welding arc characteristics. However, thepresent invention should be understood as contemplating and includingthe concept of the electronic welding station 300 having a receptacle orcavity into which the control module 340 can be inserted. Also, eventhough modules 340 are preferred so as to allow the arc characteristicsto be tailored for a variety of welding operations, the presentinvention should also be understood as contemplating and including theconcept of the potentiometers 341A-341C being hardwired into eitherwelding torch and feeder assembly 320 or electronic welding station 300.

Turn now to FIG. 9B which is an alternative embodiment of the presentinvention using a read-only memory module. The construction andoperation of the embodiment of FIG. 9B is identical to that of FIG. 9Aexcept for the following differences. In addition to power, control andlogic circuits, component 301 also contains a processor. Circuit 301 isconnected by bus 351 to connector 350. Module 353 contains a read-onlymemory 355 which is connected to connector 354. Connector 354 isdesigned to mate with connector 350 when module 353 is inserted intocavity or receptacle 352. Memory 355 may be a true read-only memory orone of the types of programmable read-only memories. Memory 355 containsinformation corresponding to the graph of FIG. 8 for a particular typeof welding operation. In the preferred embodiment, the memory 355 foreach module 353 will contain welding parameter information, such as thatdepicted by FIG. 8, for only one type of welding operation. However, thepresent invention should be understood as including and contemplatingthe concept of a single memory 355 containing welding parameterinformation for a plurality of different types of welding operations sothat a single module 353 will allow the electronic welding station 300to be used for the variety of welding operations without the necessityof changing module 353. The present invention should also be understoodas including and contemplating the concept of memory 355 being hardwiredinto, and being a part of, the processor, power, control and logiccircuits 301.

In the circuit of FIG. 9B, pulsewidth modulator 302 is directlycontrolled by processor and logic circuits 301. Similarly, wire feedspeed control circuit 323 is directly controlled by processor and logiccircuits 301 via buses 304 and 322.

The welding torch and feeder assembly 320 contains a multi-positionswitch 360 which is connected to bus 322. When the welding operatorturns control knob 361, switch 360 advises the processor in component301 of the new switch position. The processor, using the informationcontained in memory 355, then provides the appropriate new settings forthe pulsewidth modulator 302 and wire feed speed control circuits 323.For efficiency and for the convenience of the welding operator, it ispreferred that control knob 361 and switch 360 be a part of weldingtorch 320. However, it should be understood that the present inventionincludes and contemplates the concept of switch 360 and control 361being included in the electronic welding station 300.

Turn now to FIG. 10 which is an illustration of the inductor/transformer67 used in the preferred embodiment. For convenience, windings 67A, 67B,67C, and 67F are not shown. The dimensions and construction ofinductor/transformer 67 are not critical. It is sufficient that thecoupling between windings 67A, 67B, and 67C be sufficient to allowcomponent 67 to function as a transformer when plug 67E is inserted tocomplete core 67D, and that the inductance of winding 67A be reduced toallow component 67 to function as a smoothing reactor when plug 67E isremoved. Plug 67E is preferably trapezoidal in shape so as to providefor a tight fit which reduces or eliminates any air gaps between core67D and plug 67E when plug 67E is inserted. In the preferred embodiment,plug 67E is cut from the original core 67D and the cut surfaces are thenmachined so as to provide good metal-to-metal contact.

From a reading of the above, it will be understood that the presentinvention comprises improvements to an electronic welding station so asto provide for positive ground, negative ground, and AC modes ofoperation, a voltage control mode and a cleaning/penetration controlmode of AC operation, detection of the arc being struck and adjustmentof the cooling fan speed and the wire feed speed accordingly, reductionof the probability of failure of hard to access transistors, selectionof readily accessible transistors for failure in the event of overloador short circuit conditions, and use of a single selection control toautomatically select the parameters most desirable to perform aparticular type of welding operation. Although the preferred embodimentof the present invention has been described with particularity,modifications to the preferred embodiment and alternative embodimentswill suggest themselves to those skilled in the art. Accordingly, thescope of the present invention is to be limited only by the claimsbelow.

I claim:
 1. A welding station for selectably providing for positiveground operation or negative ground operation, comprising:a positivepower bus and a negative power bus for receiving input power; a positiveoutput terminal and a negative output terminal for providing an outputvoltage to a welding operation; welding parameter selection and drivemeans for providing a drive signal having parameters desired for aselected welding operation; first switching means having a firstterminal and a second terminal and responsive to said drive signal forconducting current between said first terminal and said second terminal;and second switching means for selectably providing for said positiveground operation by connecting said positive power bus to said positiveoutput terminal, connecting said negative power bus to said secondterminal, and connecting said negative output terminal to said firstterminal, and for selectably providing for said negative groundoperation by connecting said positive power bus to said first terminal,connecting said positive output terminal to said second terminal, andconnecting said negative power bus to said negative output terminal. 2.The welding station of claim 1 wherein said first switching meanscomprises a semiconductor device.
 3. The welding station of claim 1wherein said first switching means comprises a series connection of aplurality of parallel-connected semiconductor devices and an inductor.4. The welding station of claim 3 wherein, for said positive groundoperation, said second switching means connects said inductor in serieswith said first terminal of said first switching means and, for saidnegative ground operation, said second switching means connects saidinductor in series with said second terminal of said first switchingmeans.
 5. The welding station of claim 1 and further comprising a diodeconnected in series with said first terminal of said first switchingmeans for preventing a reverse current from flowing with respect to saidfirst terminal.
 6. The welding station of claim 1 wherein said weldingparameter selection and drive means comprises:welding parameterselection means for generating a first signal having said parametersdesired for said selected welding operation; and drive means responsiveto said first signal for providing said drive signal.
 7. The weldingstation of claim 6 wherein said drive means comprises means forelectrically isolating said welding parameter selection means from saidfirst switching means.
 8. The welding station of claim 1 and furthercomprising:means for determining if an arc is present for causing saiddrive signal to have a first set of characteristics if said arc isabsent and a second set of characteristics if said arc is present. 9.The welding station of claim 8 wherein said means for determiningcomprises:means for generating an absolute value of said output voltage;and means responsive to said absolute value of said output voltageexceeding a predetermined value for determining that said arc is absent.10. The welding station of claim 1 and further comprising:short circuitdetection means responsive to said output voltage for disabling saidwelding parameter selection and drive means if said output voltage isless than a predetermined value.
 11. The welding station of claim 10 andfurther comprising:a switch for enabling said welding station; and meansfor temporarily disarming said short circuit detection means when saidswitch enables said welding station.
 12. The welding station of claim 11wherein said means for temporarily disarming comprises an arc sustainingresistor.
 13. The welding station of claim 11 wherein said means fortemporarily disarming comprises a snubber circuit.
 14. A welding stationfor selectably providing for positive ground, negative ground, and ACoutput operation, comprising:a positive power bus and a negative powerbus for receiving input power; a first output point and a second outputpoint for providing for said positive ground and said negative groundoperation; a third output point and a fourth output point for providingfor said AC output operation; welding parameter selection and drivemeans for providing a first drive signal and a second drive signalhaving parameters desired for a selected welding operation; firstswitching means having a first terminal and a second terminal andresponsive to said first drive signal for conducting current betweensaid first terminal and said second terminal; second switching meanshaving a first terminal and a second terminal and responsive to saidsecond drive signal for conducting current between said first terminaland said second terminal; an output transformer having a primary windingfor receiving drive power, and a secondary winding for providing anoutput voltage and an output current to a welding operation, saidsecondary winding being connected to said third output point and saidfourth output point; third switching means for selectably providing forpositive ground operation by connecting said positive power bus to saidfirst output point and connecting said negative power bus to said secondoutput point through a first series circuit comprising said firstswitching means and a first part of said primary winding, providing fornegative ground operation by connecting said negative power bus to saidsecond output point and connecting said positive power bus to said firstoutput point through said first series circuit, and providing for ACoutput operation by connecting said first series circuit between saidpositive power bus and said negative power bus, connecting a secondseries circuit comprising said second switching means and a second partof said winding between said positive power bus and said negative powerbus; and means for connecting a welding torch and a workpiece to aselected pair of said output points for providing an output voltage andan output current to said selected welding operation.
 15. The weldingstation of claim 14 wherein, for said positive ground and negativeground operation, said first series circuit further comprises saidsecond switching means, said second switching means being connected inparallel with said first switching means.
 16. The welding station ofclaim 14 wherein said output transformer is configurable as atransformer for said AC output operation and configurable as an inductorfor said positive ground and said negative ground operation.
 17. Thewelding station of claim 16 wherein said output transformer comprises acore having a selectably removable portion, said portion being insertedto configure said output transformer as a transformer, and being atleast partially removed to configure said output transformer as aninductor.
 18. The welding station of claim 14 and further comprisingmeans for causing said second drive signal to be approximatelyequivalent to said first drive signal for said positive ground and saidnegative ground operation.
 19. The welding station of claim 14 andfurther comprising variable reactance means connected between saidsecondary winding and said third output point for controlling saidoutput current provided by said welding station during said AC outputoperation.
 20. The welding station of claim 19 wherein said variablereactance means comprises a saturable reactor.
 21. The welding stationof claim 20 wherein said saturable reactor comprises a controltransformer having a first winding connected between said secondarywinding of said output transformer and said third output point, and asecond winding connected to a variable DC power supply.
 22. The weldingstation of claim 14 wherein said welding station has an asymmetrical ACoutput mode and a pulsed AC output mode.
 23. The welding station ofclaim 22 wherein said parameter selection and drive meanscomprises:welding parameter selection means for generating a firstsignal and a second signal; first drive means responsive to said firstsignal for providing said first drive signal; second drive meansresponsive to said second signal for providing said second drive signal;and signal control means for causing said second drive signal to be aninverted version of said first drive signal when said welding station isin said asymmetrical AC output mode, and for causing said second drivesignal to be a time delayed version of said first drive signal when saidwelding station is in said pulsed AC output mode.
 24. The weldingstation of claim 23 wherein said signal control means comprises meansfor inverting said second signal when said welding station is in saidasymmetrical output mode.
 25. The welding station of claim 23 whereinsaid welding parameter selection means has a selectable oscillationfrequency, said oscillation frequency in said pulsed AC mode beingapproximately twice said oscillation frequency in said asymmetrical ACmode.
 26. The welding station of claim 14 and further comprising:meansfor determining if an arc is present for causing said first drive signaland said second drive signal to have a first set of characteristics ifsaid arc is absent and a second set of characteristics if said arc ispresent.
 27. The welding station of claim 26 wherein said means fordetermining comprises:means for generating an absolute value of saidoutput voltage; and means responsive to said absolute value of saidoutput voltage exceeding a predetermined value for determining that saidarc is absent.
 28. The welding station of claim 14 wherein saidtransformer has a core and further comprising means for controlling saidoutput current provided by said welding station during said AC outputoperation by varying a degree of saturation of said core.
 29. Thewelding station of claim 28 wherein said means for varying said degreeof saturation comprises:a control winding wound on said core; and meansfor controlling an amount of current flowing through said controlwinding.
 30. The welding station of claim 14 and further comprising adiode connected in series with said first terminal of said firstswitching means for preventing a reverse current from flowing withrespect to said first terminal.
 31. The welding station of claim 14 andfurther comprising a diode connected in series with said first terminalof said second switching means for preventing a reverse current fromflowing with respect to said first terminal.
 32. The welding station ofclaim 14 wherein:said primary winding of said transformer has a firstend and a second end, and a tap point connected to a source of power,said first part of said primary winding being between said first end andsaid tap point and said second part of said primary winding beingbetween said second end and said tap point.
 33. The welding station ofclaim 32 wherein said tap point is located such that said first part ofsaid primary winding has a different number of turns than said secondpart of said primary winding.
 34. The welding station of claim 14 andfurther comprising:short circuit detection means responsive to saidoutput voltage for disabling said welding parameter selection and drivemeans if said output voltage is less than a predetermined value.
 35. Thewelding station of claim 34 and further comprising:a switch for enablingsaid welding station; and means for temporarily disarming said shortcircuit detection means when said switch enables said welding station.36. The welding station of claim 35 wherein said means for temporarilydisarming comprises a snubber circuit.
 37. The welding station of claim35 wherein said means for temporarily disarming comprises an arcsustaining resistor.
 38. A welding station having at least two AC modesof operation, comprising:control means for designating a selected modeof operation from a mode group comprising an asymmetrical AC output modeand a pulsed AC output mode; welding parameter selection and drive meansresponsive to said control means for providing a first drive signal anda second drive signal, said first drive signal and said second drivesignal each having parameters desired for said selected mode ofoperation, said second drive signal being an inverted version of saidfirst drive signal when said asymmetrical AC output mode is selected andsaid second drive signal being a time delayed version of said firstdrive signal when said pulsed AC output mode is selected; a transformerhaving a primary winding for receiving drive power and a secondarywinding for providing an output voltage and an output current to awelding operation; and switching means responsive to said first drivesignal and said second drive signal for providing said drive power tosaid primary winding of said transformer.
 39. The welding station ofclaim 38 wherein:said primary winding of said transformer has a firstend and a second end, and a tap point connected to a source of power;and said switching means comprises a first transistor and a secondtransistor, said first transistor being connected to said first end andresponsive to said first drive signal for driving said transformer, saidsecond transistor being connected to said second end and responsive tosaid second drive signal for driving said transformer.
 40. The weldingstation of claim 38 wherein:said primary winding of said transformer hasa first end, a second end, and a tap point, said tap point being locatedsuch that said primary winding is divided into two unequal sections,said tap point being connected to a source of power; and said switchingmeans comprises a plurality of first transistors and a secondtransistor, said plurality being connected to said first end andresponsive to said first drive signal, said second transistor beingconnected to said second end and responsive to said second drive signal.41. The welding station of claim 38 and further comprising a variablereactor connected in series with said secondary winding for limitingsaid output current.
 42. The welding station of claim 41 wherein saidvariable reactor comprises:a second transformer having a first windingconnected in series with said secondary winding of said outputtransformer, a core, and a second winding; and means connected to saidsecond winding of said second transformer for controlling a degree ofsaturation of said core of said second transformer.
 43. The weldingstation of claim 38 and further comprising:short circuit detection meansresponsive to said output voltage for disabling said welding parameterselection and drive means if said output voltage is less than apredetermined value.
 44. The welding station of claim 43 and furthercomprising:a switch for enabling said welding station; and means fortemporarily disarming said short circuit detection means when saidswitch enables said welding station.
 45. The welding station of claim 44wherein said means for temporarily disarming comprises a snubbercircuit.
 46. The welding station of claim 38 wherein said weldingparameter selection and drive means comprises:welding parameterselection means for providing a first signal and a second signal; firstdrive means, responsive to said first signal when said pulsed AC outputmode is selected and to said first signal and said second signal whensaid asymmetrical AC output mode is selected, for providing said firstdrive signal; and second drive means, responsive to said second signalwhen said pulsed AC output mode is selected and to said first signal andsaid second signal when said asymmetrical AC output mode is selected,for providing said second drive signal.
 47. The welding station of claim46 wherein said second drive means provides said second drive signal bycombining said first signal and said second signal to produce a thirdsignal, and inverting said third signal when said asymmetrical AC outputmode is selected.
 48. The welding station of claim 46 wherein saidwelding parameter selection means comprises an oscillator having a firstoscillation frequency when said asymmetrical AC output mode is selectedand a second oscillation frequency when said pulsed AC output mode isselected.
 49. The welding station of claim 38 and furthercomprising:means for determining if an arc is present for causing saidfirst drive signal and said second drive signal to have a first set ofcharacteristics if said arc is absent and a second set ofcharacteristics if said arc is present.
 50. The welding station of claim49 wherein said means for determining comprises:means for generating anabsolute value of said output voltage; and means responsive to saidabsolute value of said output voltage exceeding a predetermined valuefor determining that said arc is absent.
 51. The welding station ofclaim 38 wherein said transformer has a core and further comprisingmeans for controlling said output current provided by said weldingstation during said AC modes of operation by varying a degree ofsaturation of said core.
 52. The welding station of claim 51 whereinsaid means for varying said degree of saturation comprises:a controlwinding wound on said core; and means for controlling an amount ofcurrent flowing through said control winding.
 53. An apparatus which isuseable as an inductor or as a transformer, comprising:a metallic corehaving a first part and a second part, said second part beingselectively removable from and insertable into said first part, saidmetallic core forming only a single path for a magnetic field when saidsecond part is inserted into said first part; a first winding on saidcore, said winding having a first self-inductance when said second partis removed from said first part and a substantially largerself-inductance when said second part is inserted into said first part;and a second winding on said core, said winding having a secondself-inductance when said second part is removed from said first partand a substantially larger self-inductance when said second part isinserted into said first part; said first winding and said secondwinding have a first degree of coupling when said second part is removedfrom said first part and a substantially larger degree of coupling whensaid second part is inserted into said first part; whereby saidapparatus functions effectively as a transformer when said second partis inserted into said first part and functions primarily as an inductorwhen said second part is removed from said first part.
 54. The apparatusof claim 53 wherein:said first part has two angled faces; said secondpart has two angled faces which match said angled faces of said firstpart; and said faces for said first part and said faces for said secondpart are machined surfaces so as to provide for good metal-to-metalcontact when said second part is inserted into said first part.
 55. Theapparatus of claim 53 wherein:said second part is shaped like a wedge.56. The apparatus of claim 53 and further comprising:a third winding onsaid core, said third winding being connected to a DC current sourcewhich provides a selectable current to said third winding; whereby saidselectable current and said third winding control a degree of saturationof said core and thereby control said self-inductance of said firstwinding and said second winding and said coupling between said firstwinding and said second winding.
 57. The apparatus of claim 53wherein:said first winding is a tapped primary winding; and said secondwinding is a secondary winding.
 58. The apparatus of claim 53 whereinsaid first winding has a different number of turns than said secondwinding.